Facts about Mast Cell Tumors Dog Owners Should Know


There are various types of tumors that affect dogs, but there is one in particular, the mast cell tumor, which can behave quite erratically and that deserves a dog owner’s attention. Dog owners should keep their vigilant antennas up and pay attention when petting and grooming their dogs as these tumors are not only quite common but also quite unpredictable in nature. Mast cell tumors, also known as mastocytomas, are one reason why, even the most innocent looking bump or lump should be checked out by a veterinarian. This is why it’s never really a good idea to take a wait and see approach with any lumps and bumps, unless a veterinarian has determined that it’s safe to do so. So today, let’s take a look at some surprising facts about mast cell tumors in dogs.

mast-cell-tumorMast Cell Tumors are Copy Cats…

Mast cell tumors in dogs are often referred to as “the great imitators,” why is that? Mast cell tumors gain this reputation from the fact that they can clinically resemble many other types of dog skin tumors. (See pictures for an idea)

Mast cell tumors may therefore look like an innocent bump, a fatty mass under the skin, an ugly ulcerated mass or a bug bite. You name it! Mast cell tumors in dogs can also be smooth, bumpy, solitary or in groups and they may be present on the skin or underlying tissues.

This is again why, one can’t never say what a mass really is until it gets checked out. However, despite having a variety of clinical appearances, if one must describe how a mast cell in a dog looks like on average, a mast cell tumor  can be described as a hairless, pink, raised mass that prefers to show up on the dog’s torso and legs, explains  Steven Neihaus, a board-certified veterinary surgeon.

“Mast cell tumors can occur anywhere on the body. Approximately 50% occur on the trunk, 40% on limbs, and 10% on the head. ” Source, DVM360


And They Can Play Peek-a-Boo too.mast-cell

Dog mast cell tumors can be quite unpredictable tumors. For instance, some mast cells tumors may have a history of shrinking for some time and then swelling up again.

The shrinking may lead dog owners to assume that the growth is something that is getting smaller which may cause them to delay treatment. Then, after a while they get a wake-up call once the growth starts swelling up again.

This history of shrinking and then swelling is due to the mast cell tumor’s tendency to degranulate and release histamine, explains Tracy Geiger, a board-certified veterinarian specializing in internal medicine.


mast-cell-cancerMast Cell Tumors Release Substances…

Mast cell tumors don’t just sit there all day long doing nothing. These tumors originate from the bone marrow but then finish up maturing in the tissues of a dog’s body and this can includes skin, digestive tract and respiratory tract.

Once stimulated by the immune system, mast cells tumors release their granular contents along with several chemicals and these may include histamine, proteoglycans, neutral proteases (enzymes that break down protein) and chemotactic growth factors.

Histamine in particular, is basically the same stuff that causes an area of the skin affected by a bee stings to become red, inflamed and painful.When all these chemicals are released, they can, not surprisingly, wreck quite some have havoc on a dog’s body.

That Can Cause Complicationsveterinary

As mentioned, mast cell tumors are quite insidious in nature causing a variety of problems when they release chemicals.

Among dog mast cell tumor complications, at a skin level, recurrent swelling may occur due to degranulation and associated release of histamine, while local bruising, this time from the release of heparin, may be present as well.

When mast cells tumors release histamine into the dog’s bloodstream, they may trigger the dog’s stomach lining to produce too much acid and this may lead to a decrease in appetite, nausea, lip smacking, drooling and vomiting, explains veterinarian Dr. Dressler.

Because of these dog mast cell tumor complications, anti-acids are often prescribed to manage the excess acid production, while antihistamines are used to block the  release of histamine allowing the body a better chance of coping with the high histamine levels. Both cancerous and non-cancerous forms of mast cell tumors may release histamine.

” Dogs can also develop signs associated with the release of toxins from the malignant mast cells. For example, up to a quarter of dogs with mast cell tumors also have stomach ulcers due to histamine release.” ~Merck Veterinary Manual

dog-mast-cell-tumor-cellsThey Can be Quite Easily Diagnosed

In the case of a suspected lump or bump, it can be aspirated with a fine needle to determine whether the growth is cancerous or not. The needle aspiration is done with a small gauge needle and shouldn’t be painful hence, the term “fine needle aspiration.”

The fine needle aspirate cells can then be evaluated under a microscope (cytology) in house or  the sample can be sent out to be evaluated by a pathologist.

Under a microscope, the sample typically shows a large number of mast cells which is enough to make a diagnosis of mast cell tumor. Mast cells show up as purples granules that contain histamine.Once mast cell tumor is confirmed, a surgical biopsy is needed to discover the grade of the tumor.

“Diagnosis can often be made with a needle aspirate, which collects some cells of the tumor with a needle, and the cells are examined under the microscope. The granules have distinct staining characteristics leading to their recognition. An actual tissue biopsy, however, is needed to grade the tumor and grading of the tumor is crucial to determining prognosis.”~Dr. Wendy C. Brooks

But Removal Requires Wide Marginsdog-surgery

If you think that surgery to remove a mast cell tumor involves just simply removing only the the lump or bump, think again.

In order to get rid of this pesky tumor, wide margins are required. Because it’s difficult to tell where the tumor begins and where it ends, a large area of about 3 inches of ‘healthy’ tissue in all directions must be removed. So this explains why dog mast cell tumor need wide margins.

Getting wide mast cell tumor margins though can sometimes be a problem depending on where the tumor is located. For instance, mast cell tumors on the neck or in the mouth may be problematic.

Should the mast cell tumors have metastasized to other areas, a combo of anti-cancer drugs may be used along with surgery and radiation. For the best treatment plan, it’s often a good idea to consult with a veterinary oncologist.

The Bottom Line

Here are some more dog mast cell tumor facts: mast cell tumors in dogs account for up to 20 percent of all tumors affecting the skin in dogs. While they may mostly affect older dogs, they can be found in dogs of any age dog. Mast cell tumors may also affect any breed,  even though certain breeds such as boxers, pugs, Boston terriers, bulldogs, bull terriers, and retrievers are known for being predisposed. As with other types of tumors, it’s important to promptly report to the vet any suspicious growth, lump or bump for the best possible outcome.

Disclaimer: this article is not meant to be used as a substitute for professional veterinary advice. If your dog has a growth on his skin, please consult with your vet promptly for proper diagnosis and treatment.


  • Pet Education: Canine Mast Cell Tumors, retrieved from the web on November 19th, 2016.
  • Dog Cancer Blog, Why use stomach medication for mast cell tumors? retrieved from the web on November 19th, 2016.
  • Dog Cancer Blog, Why Benadryl For Mast Cell Tumors? retrieved from the web on November 19th, 2016.
  • DVM360,  Mast cell tumors in dogs and cats, retrived from the web on November 18th, 2016

Photo Credits:

I am Your Dog’s Mast Cells

Your dog’s immune system is always working around the clock to keep your dog protected from insidious diseases. In particular, white blood cells, the cells of your dog’s immune system, play the role of soldiers actively protecting the fortress (your dog’s body) against infectious diseases and foreign invaders. Mast cells, even though having a bad rap due to allergies, anaphylaxis and cancer, are a type of white blood cell which also have a protective role. Today, let’s take a closer look into a dog’s mast cells, what they do, how they work and the things that can wrong.

dog-mast-cellsIntroducing Your Dog’s Mast Cells 

Hello, it’s your dog’s mast cells talking! As mentioned, we are a type of white blood cell. Actually, to be more descriptive, we are granulocytes, a sub-type of white blood cells that are known for having granules in their cytoplasm. Many of our granules are rich in histamine and heparin, an anticoagulant, along with other substances. We were first described by Paul Enrich in 1878, who noted our distinguishing feature of having large granules.

We are born in your dog’s bone marrow and then distribute in most tissues of your dog’s body, but we’re mostly like to stick around the skin, digestive tract, respiratory tract, mouth, nose and eye area. In healthy dogs, we normally populate body tissues only, and only very rarely are we detected into the blood stream.

We are equipped with special storage sacs which house powerful molecules called mediators which are released in specific circumstances and that produce local responses.

We Stand Guarddog-guarding-home

As mentioned, we are guardians who protect the body from perceived invaders and actively respond to the presence of allergens and inflammation. Since we are located by the skin, mouth, eyes and nose, we play a prime role in detecting allergens. We are basically pretty inactive beings when all is well, but the moment we detect an allergen in your dog’s body, we rapidly degranulate, releasing histamine. This explains why your dog gets all itchy when he eats a food he’s allergic to!

When histamine is released, it can lead to swelling, redness, itching, welts and even anaphylaxis, a severe systemic reaction to an allergen such as from bees stings and drugs. In dogs, skin allergies are more common rather than the classic allergy symptoms seen in humans such as sneezing and developing a watery nose and eyes. Antihistamines are helpful in keeping allergy symptoms under control.

“Histamine is useful in the body at certain levels.  Mast cells release histamine which helps attract other white cells to an area or an invader to help clean up the area, or mount an immune system reaction…  Histamine is important in immunity. However, too much histamine is not good for the body.” ~ Dr. Demain Dressler

When Things Go Wrongveterinary

Yes, there can be too much of a good thing. While we are master immune regulators and infection fighting cells, things can get out of hand at times, such as when there are too many of us or we “misbehave” causing severe symptoms and disorders.

Mast Cell Tumors

We are probably mostly known for wrecking trouble as it happens with mast cell tumors.  Fortunately, malignant mast cell cancer can be often ruled out or confirmed through a fine needle aspirate. Remember how we said earlier that we rarely appear in the bloodstream? Well,  when a mast cell tumor is highly malignant, it may spread and appear in the blood stream. An advanced mast cell tumor can therefore be detected with a blood test by looking at the “buffy coat.

When it comes to the malignant, aggressive forms, mast cell tumors are often referred to as “the great imitators,” because it can take many different forms which can be confused with other skin problems. Sometimes they may present as a solitary bump or in groups on the dog’s skin, and often look like innocuous masses. This is why when your dog develops any unusual looking lumps, bumps or lesions, you should have them checked out by the vet, especially if they grow and change appearance. Mast cell skin tumors may appear as lumps that stay the same size for a while and then start growing. Mast cell tumors can be seen in any dogs, but boxers and Boston terriers are particularly affected.

When caught early and affecting only the skin (grade 1) mast cell tumors can be surgically removed, with wide and clean margins, and the cancer can be kept at bay.  Things start getting progressively worse through with grade 2 and 3. Treatment options therefore vary based on the grade of this tumor. Because we release histamine, heparin, and other enzymes when we’re damaged, we can cause major disruptions.  Too much histamine can cause the dog’s stomach to produce too much acid, which is why antihistamines and antiacids are often prescribed to dogs with mast cell tumors. On top of increased acidity, the release of histamine, heparin, and other enzyme may also have negative effects on the dog’s heart rate, blood pressure, and other important body functions.

While mast cell tumors most commonly affect the dog’s skin (cutaneous mast cell tumors) at times, they may even affect internal organs (visceral mast cell tumors) which can produce vague symptoms such as loss of appetite, vomiting and lethargy. Skin mass cell tumors may also affect the skin only at first and then metastasize to internal organs as they advance and spread.

“It is not possible to tell with the naked eye alone whether a specific skin growth is a malignant mast cell tumor or not, and therefore, some diagnostic tests are always necessary to confirm whether a skin growth is a mast cell tumor.”~ Etienne Cote

idea tipDid you know? Mast cell tumors commonly affect a dog’s skin and subcutaneous tissues and account for 7 to 21 percent of dog skin tumors. (Source, References  1)



  • Pergamon Press Ltd. 1992, “Skin tumors of the dogs and cat,” by M.H. Goldschmidt & F.S. Shofer
  • Clinical Medicine of the Dog and Cat, Second Edition, edited by Michael Schaer
  • Mast Cell Tumors in Dogs, TEXTBOOK OF VETERINARY INTERNAL MEDICINEClient Information Series Mona P. Rosenberg, DVM DACVIM
  • Pet Education, Mast Cell Tumors, retrieved from the web on October 24th, 2016

Photo Credits:

Kidz Search, author Kauczuk, Photo of skin mast cells at 100X using an oil immersion lens and an olympus digital camera. The cells are stained with Tol Blue, and might appear slightly degranulated as they were activated using an artificial antigen during the course of an experiment.



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